Stirrer mills for the continuous fine grinding and/or dispersion of, in particular, solid materials in liquids are for example known from U.S. Pat. No. 2,855,156; in this connection it must be borne in mind that as a rule the part played by dispersion on the one hand and fine grinding on the other hand in the course of a process may vary, that is to say in certain cases dispersion may be more important and grinding less important, or vice versa.
Stirrer mills of this kind usually consist of an upright or horizontal grinding vessel in which a stirrer, usually composed of discs, rotates at high speed. The grinding elements used may be a filling of sand or of steel or glass beads or the like. The material to be ground is pumped to the grinding vessel of the stirrer mill at one end, leaving the mill at the other end after being separated from the grinding elements. Experience has shown that predispersed pumpable material to be ground must be fed to stirrer mills. For the premixing or predispersion use is generally made of mixing machines or stirrers of large volume in which, for example, the solids and liquids are brought into contact with one another. Apart from the enormous expense for apparatus entailed by such premixing machines, the quality of the premixing has been shown by experience not to be sufficiently effective to prevent the occurrence of a considerable quantity of solid agglomerates of substantial size, since in mixers of this kind the components of the mixture are acted on only with statistical probability. After the mixing process the mixture of liquid and material to be ground is fed through metering pumps and pipes to the stirrer mill. Because of the previously mentioned solid agglomerates, however, the performance of the stirrer mill is considerably reduced, quite apart from the resulting difficulties in the metering pumps and pipes.
From German Patent No. 1,227,767 it is already known to separate a premixing chamber in the bottom region of the vessel from the actual mixing chamber by means of a sieve. The dispersion liquid and solid materials are each introduced separately into this premixing chamber, while metering worms may be provided for the introduction of the solids. The premixing is effected by means of mixing tools mounted on an extension of the stirrer shaft, or by means of a separate and separately driven mixing unit. It has been found that the solids briquette or agglomerate in the conveyor worms, since as a rule considerable pressures occur in the latter, if only because conveying is effected against the static pressure of the stirrer mill. These agglomerates cannot be adequately comminuted in the premixing chamber, so that known stirrer mills do not work satisfactorily. Furthermore, the dividing sieve which is provided between the premixing chamber and the grinding chamber, and which, because of the usually small diameter of the grinding balls (0.3 to 3mm), must be a fine mesh sieve, is clogged by these agglomerates, so that the stirrer mill is then subject to frequent breakdowns. Furthermore, clogging often occurs in these conveyor worms, so that the latter no longer operate but are brought to a halt.
From German Patent No. 1,249,649 it is known for the material to be ground, which is in paste form and is not mixed with an additional dispersion medium, to be delivered by means of a conveyor worm to the grinding chamber of a stirrer mill.